Noise suppression system

ABSTRACT

A noise suppression system for an air-cooled internal combustion engine is disclosed. The system may include an acoustically designed shroud forming a cavity and configured for attenuating noise produced by a cooling air fan associated with the engine. In one system, the shroud is mountable on a housing of the air fan. A pair of air inlet passages may be provided which are operable to draw ambient cooling air into the shroud to the fan. The air inlet passages are acoustically configured and tuned to attenuate noise generated by the fan. In one system, the air inlet passages may each be formed in a rear quadrant of the shroud. Various configurations of the shroud may include quarter wave resonators and/or micro-perforated panels to further attenuate fan noise.

BACKGROUND OF THE INVENTION

The present invention generally relates to sound or noise suppression,and more particularly to attenuate cooling system noise pertaining toair cooled internal combustion engines.

Air cooled internal combustion engines are used to power outdoorequipment for a variety of applications including riding mowers,tractors, and others. Fans (sometimes referred to as “blowers”) used inengine cooling systems may be a source of noise.

A noise suppression system for an engine is desirable.

SUMMARY OF THE INVENTION

A noise suppression system for an air cooled engine is disclosed thatattenuates noise generated by operation of the engine's cooling fan orblower. In one aspect of the disclosure, the noise suppression systemincludes an acoustically designed shroud which in one configuration maybe mounted on an air blower housing. The shroud may further beconfigured to define a cooling air inflow path for drawing ambientcooling air inwards towards the fan. The shroud may include any numberof elongated air inlet passages (also referred to as “chambers”) whichare in fluid communication with the engine's fan and sonicallyconfigured to reflect and attenuate fan noise.

As disclosed herein, various shrouds may further include other noisesuppression features including without limitation quarter waveresonators, micro-perforated panels, and fibrous materials which may beused alone or in combination to add to fan noise attenuation.

According to one aspect of the present disclosure, a noise suppressionshroud for an engine includes a body defining a longitudinal axis andhaving a lower portion configured for mounting on a cooling air fan ofthe engine and an upper portion, a cavity formed in the body, and aquarter wave resonator disposed in the cavity and tuned to attenuatenoise generated by the fan within or at a first range of frequencies,such as at the primary blade-pass frequencies. The quarter waveresonators may be comprised of a plurality of intersecting partitionsforming a plurality of corresponding cells. In some shrouds, at leastone micro-perforated panel may be disposed in the shroud which is tunedto attenuate (and/or otherwise reduce) noise within or at a second rangeof frequencies, such as a larger range of blade-pass frequencies. Invarious shrouds, the shroud may include any number of horizontallyelongated air inlet passages which are in fluid communication with a fanimpeller positioned in a top of the cooling air fan. A majority of eachair inlet passage may be positioned in one of two rear quadrants definedby the shroud adjacent the rear of the engine.

According to another aspect of the present disclosure, a noisesuppression system for an engine includes a shroud defining alongitudinal axis and configured for mounting on a cooling air fanassociated with the engine, a cavity formed in the shroud, and at leastone micro-perforated panel disposed in the shroud. The micro-perforatedpanel may be tuned to reduce noise generated by the fan within or at afirst range of frequencies. Some noise suppression systems may furtherinclude a second micro-perforated panel disposed in the shroud. Thesecond micro-perforated panel may be used to increase the effectivefrequency range of the micro-perforated panels.

According to another aspect of the present disclosure, a noisesuppression system for an air cooled engine includes a shroud defining alongitudinal axis and configured for mounting on a housing of a coolingair fan associated with the engine, a cavity formed in the shroud andpositioned over a fan impeller rotationally supported by the housing ofthe cooling air fan, and a pair of horizontally elongated air inletpassages each being formed in a rear quadrant of the shroud. The airinlet passages are in fluid communication with the cavity and operableto draw ambient cooling air into the shroud in a forward directiontowards the fan impeller. The air inlet passages are acousticallyconfigured to attenuate noise generated by the cooling air fan. In onesystem, the air inlet passages may include a plurality of angledsidewall surfaces configured for reflecting noise generated by the fanimpeller.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the preferred embodiments will be described withreference to the following drawings where like elements are labeledsimilarly, and in which:

FIGS. 1A and 1B are side elevation and top plan views respectively ofpower equipment having an engine incorporating a noise suppressionsystem according to the present disclosure;

FIG. 2 is a front top perspective view of the cooling air blower ofFIGS. 1A and 1B with noise suppression shroud;

FIG. 3 is bottom front perspective thereof;

FIG. 4A is top plan view thereof;

FIG. 4B is a transverse front cross-sectional view thereof;

FIG. 4C is a longitudinal side elevation cross-sectional view thereof;

FIG. 5 is side elevation view thereof;

FIG. 6 is front elevation view thereof;

FIG. 7 is a bottom plan view thereof;

FIG. 8 is a bottom perspective view thereof;

FIG. 9 is a top perspective view of the blower housing with shroudremoved;

FIG. 10 is a top plan view thereof;

FIG. 11 is a bottom plan view thereof;

FIG. 12 is a front perspective view of the shroud;

FIG. 13 is a front view thereof;

FIG. 14 is a bottom plan view thereof showing a quarter wave resonatorinside the shroud;

FIG. 15 is a top plan view of the shroud;

FIG. 16 is a side elevation thereof;

FIG. 17 is bottom rear perspective view thereof;

FIG. 18 is a longitudinal side elevation cross-sectional view thereof;

FIG. 19 is a front perspective view of a shroud base;

FIG. 20 is a bottom rear perspective view thereof;

FIG. 21 is a top plan view thereof;

FIG. 22 is a front elevation view thereof;

FIG. 23 is a side elevation view thereof;

FIG. 24 is a rear elevation view thereof;

FIG. 25 is a front perspective view of the shroud base and coverassembly;

FIG. 26 is a side elevation cross-sectional view of the shroud;

FIG. 27 is a bottom plan view of the shroud with a second configurationof a quarter wave resonator;

FIG. 28 is a bottom plan view of the shroud with a micro-perforatedpanel;

FIG. 29 is a longitudinal side elevation cross-sectional view thereof;

FIG. 30 is longitudinal side elevation cross-sectional view of a shroudhaving two micro-perforated panels;

FIG. 31 is top plan view of a mono-pitch air blower impeller usable inthe cooling air blower of FIG. 2 having blades which are equally spacedapart;

FIG. 32 is a cross-sectional view thereof;

FIG. 33 is a bottom plan view thereof;

FIG. 34 is a side elevation view thereof;

FIG. 35 is a bottom plan view of a modulated pitch air blower impellerusable in the cooling air blower of FIG. 2 having blades which areunequally spaced apart showing three different sinusoidal modulations;

FIG. 36 is a cross-sectional side elevation view thereof;

FIG. 37 is a side elevation view thereof;

FIG. 38 is a bottom plan view thereof; and

FIG. 39 is a graph showing sound transmission loss predictive modelingresults

All drawings are schematic and not necessarily to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

The features and benefits of the present disclosure are illustrated anddescribed herein by reference to exemplary embodiments. This descriptionof exemplary embodiments is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. Accordingly, the present disclosure expresslyshould not be limited to such embodiments illustrating some possiblenon-limiting combination of features that may exist alone or in othercombinations of features; the scope of the claimed invention beingdefined by the claims appended hereto.

In the description of embodiments disclosed herein, any reference todirection or orientation is merely intended for convenience ofdescription and is not intended in any way to limit the scope of thepresent invention. Relative terms such as “lower,” “upper,”“horizontal,” “vertical,”, “above,” “below,” “up,” “down,” “top” and“bottom” as well as derivative thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) should be construed to refer to theorientation as then described or as shown in the drawing underdiscussion. These relative terms are for convenience of description onlyand do not require that the apparatus be constructed or operated in aparticular orientation. Terms such as “attached,” “coupled,” “affixed,”“connected,” “interconnected,” and the like refer to a relationshipwherein structures are secured or attached to one another eitherdirectly or indirectly through intervening structures, as well as bothmovable or rigid attachments or relationships, unless expresslydescribed otherwise. The terms “sound” and “noise” may be usedinterchangeably herein unless specifically noted to the contrary.

FIGS. 1A and 1B show an exemplary piece of power equipment which mayinclude a noise suppression system according to the present disclosure.In this non-limiting example, the power equipment may be a riding mower20 comprised of a frame 21 with mowing deck 22, a seat 23 for anoperator OP, wheels 25, and an engine 26 which provides the motive forceto propel the mower along a surface and operate a rotating mowing blade(not shown) housed in the mowing deck. In this type of power equipment,the operator 25 may be positioned forward of the engine. The engine 26may be any type of internal combustion engine operated on gasoline,diesel, or another suitable liquid or gaseous fuel source. While theengine 26 is shown in one orientation with inlet passages 110 directedaway from an operator OP, in other systems, the engine 26 may be rotatedabout a vertical axis such that the inlet passages 100 may be positionedin other ways. Additionally or alternatively, in other systems, theengine 26 may be used with various other power equipment or systems,such as walk-behind lawn mowers, generators, pressure washers, or aircompressors.

Referring to FIGS. 2-8, the engine 26 may be an air cooled engineincluding a fan (or blower) 30 and blower housing 40. The fan 30 and/orblower housing 40 may be mounted with (such as on top of) the engine(not shown in these figures for clarity). These figures show the fan 30,associated appurtenances, and a noise suppression shroud 100 to befurther described herein.

The fan 30 may include, or be housed within, a blower housing 40. Theblower housing 40 may be configured and dimensioned to receive andsupport a rotatable impeller 31 of the fan 30 comprised of a pluralityof blades 32 which operates to draw in ambient air and distribute thecooling air flow over the engine 26. The housing 40 may define alongitudinal axis LA, front 49 a, rear 49 b, sides 49 c, and an interiorspace 41 configured to house impeller 31 and may include portions sizedat least slightly larger than the outside diameter of impeller 31 in thehorizontal/lateral direction to define an airflow path, which willbecome apparent upon further description herein. The impeller 31 mayrotate inside the housing 40 and be powered by a mechanical coupling tothe drive shaft of engine 26. The blower housing 40 may be mounteddirectly onto the top of the engine 26 such as with threaded fastenersor another suitable coupling system. An air cleaner unit 29 may beprovided which in some units may be positioned to the rear of the blowerhousing 40.

Any suitable type of fan impeller 31 may be provided. FIGS. 31-34 showsfan impeller 31 in the configuration of a mono-pitch design havingblades 32 which are equally spaced around the circumference of theimpeller. Fan impeller 31 with equally spaced blades 32 may generate orotherwise create fan noise that is concentrated over a small band offrequencies. FIGS. 35-38 shows an alternative embodiment of a fanimpeller 33 in the configuration of a modulated design having blades 32which are unequally spaced around the circumference of the impeller andhave different sinusoidal modulations in the blade spacing. One impeller33 design may have three different sinusoidal modulations in the bladespacing. Fan impeller 33 with blades 32 of different spacings maygenerate or otherwise create fan noise that is less concentrated thanthe mono-pitched fan impeller 31, but over a wider band of frequencies.Other impellers may have more or less sinusoidal modulations in bladespacing or non-sinusoidal modulations in blade spacing.

Fan impellers 31 and 33 may each include an annular or ring-shaped bodyhaving circumferentially extending lateral sides 34, a top 35, amounting flange 38, and a bottom 36 which is positioned closest toengine 26 when the blower housing 40 is mounted thereon. Blades 32 mayextend axially between the top and bottom 35, 36 at the periphery of theimpellers 31, 33. The blades 32 may extend radially outwards from a hub37 defining an axis of rotation. The lateral sides 34 may besubstantially open as shown. In operation, cooling air may be drawndownwards through the top 35 of the impeller 31 or 33 and dischargedradially outwards through lateral sides (outer diameter) 34 of theimpellers by the blades 32 at least partially within the confines of theblower housing 40. A circumferentially extending gap 42 may be formed ininterior space 41 of the blower housing 40 between impellers 31 or 33and the inside of the housing which define an outlet air flow pathwayfor receiving cooling air from the fan 30, as further described herein.

Hereafter, reference will be made only to impeller 31 for convenienceand brevity recognizing that impeller 33 may alternatively be usedunless explicitly mentioned otherwise.

FIGS. 9-11 show the blower housing 40 and impeller 31 alone withoutnoise suppression shroud 100.

Blower housing 40 further includes a top 43, at least partially openbottom 44, and peripheral sidewalls 45 extending vertically between thetop and bottom which terminate at a bottom edge 46. Top 43 and sidewalls45 define the interior space 41 in which impeller 31 is disposed. Someblower housings 40 may have a somewhat overall trapezoidal shape in topplan view to generally complement and conform to the shape of the engine26. In the non-limiting example of the engine 26 described herein, theengine may be an air cooled vertical shaft, V-twin cylinder arrangementof any suitable horsepower (HP) for the intended application.Accordingly, the engine cylinders 27 may be disposed horizontally and atan angle to each other wherein the blower housing 40 may be providedwith a substantially conforming configuration as shown.

In some blower housings 40, an open-centered air cleaner frame 48 may beprovided at the rear of the housing which receives at least partiallytherein a portion of the air cleaner 29. The frame 48 may be configuredto complement the shape of the air cleaner.

Blower housing 40 may further include an airflow scroll shield 47disposed in interior space 41 of the housing. The scroll shield 47assists with developing a desired air flow path within the blowerhousing from impeller 31 to optimize engine cooling. Scroll shield 47 isaffixed to the blower housing and positioned between interior portionsof the sidewalls 45 and impeller 31 depending on which impeller is used.Scroll shield 47 is spaced apart from the lateral sides 34 of theimpeller in the lateral/horizontal direction. In one blower housing 40,scroll shield 47 extends circumferentially around the impeller 31 fromthe front portion of the impeller rearwards beyond the impeller. Thescroll shield 47 may be configured in a horizontally undulatingconfiguration being unequally spaced from the impeller to direct coolingair from the impeller rearwards and downward to the two cylinders 27(shown schematically in dashed lines in FIG. 5) of the engine 26. Thecooling air flows through cooling fins on each cylinder to dissipateheat generated by operation of the engine.

According to one aspect of the present disclosure, a noise suppressionsystem is provided to attenuate sound produced by cooling fan 30, theassociated cooling air system, and other engine noise propagatingthrough the blower housing 40. The noise suppression system may includea noise suppression shroud 100 which is configured and operable toattenuate and reduce noise emissions from the fan and cooling system(and other engine components) during operation of engine 26, as furtherdescribed herein. While the description may refer to attenuating,damping, and reducing noise emissions from the fan 30 and coolingsystem, it should be recognized that the noise suppressions shroud 100also operates to attenuate, damp, or reduce various other noiseemissions (such as engine noise emissions) that exist or propagatethrough the blower housing 40 or the noise suppression shroud 100.

FIGS. 12-30 show shroud 100 and various appurtenances, as furtherdescribed herein.

Shroud 100 may have a three-dimensional shell-shaped body and generallyinclude a front 101, rear 102, and opposing lateral sides 103. Shroud100 may be removably mounted on top of blower housing 40 by any suitablemethod or combinations of methods including without limitationfasteners, snap fit, frictional fit, adhesives, welding, brazing, etc.The shroud 100 may have a complementary shape which generally conformsto the shape of housing 40. Shroud 100 may further include a top wall104 and sidewalls 105 on the front 101, rear 102, and sides 103extending downwards from the top wall. The sidewalls 105 may begenerally vertical or may have different shapes, positions, ordimensions. The bottom edges of sidewalls 105 may define an open bottom108 of the shroud 100 and corresponding downwardly open internal cavity106 designed for noise suppression, and for holding additional noisesuppression features and to define a cooling air inflow path to the fan30, as further described herein.

The top wall 104 of the shroud 100 may, in some systems, be generallyhorizontal. In other systems, the top wall 104 may be slightly curved,domed or convex shaped to varying degrees, as shown by the dashed topwall 104′ in FIG. 18. In some configurations, this slightly rounded sideprofile of the top wall may provide better acoustic sound attenuationperformance that a flat top wall 104.

The dome-shaped shroud 100 and top wall 104, as well as the cavity 106that it forms, provide noise attenuation. Due to the construction andconfiguration of the top wall 104, acoustic cancelation occurs assound/noise waves reflect from surfaces and are re-directed back towardsmatching waves. Sound waves in opposite directions with equal or closefrequencies will tend to cancel each other (attenuation). Accordingly, adomed or slightly curved top wall 104 may be useful in providing noisereduction for the system. The domed or slightly curved top wall 104 mayadditionally provide increased structural support and integrity to thetop of the shroud 100, which may increase durability of the shroud 100.

The body of the shroud 100 may be a two-piece unit comprised of a lowerportion such as mounting base 113 configured for attachment onto airblower housing 40 and an upper portion such as cover 112 configured forattenuating sound. Mounting base 113 may be attached to blower housing40 by any suitable method or combinations of methods including withoutlimitation fasteners, snap fit, frictional fit, adhesives, welding,brazing, etc. Cover 112, in turn, may be removably attached to mountingbase 113 by the same foregoing methods or others. The cover 112 may beconfigured and dimensioned in some shrouds to be at least partiallyinsertable into the mounting base 113. Mounting base 113 may bevertically shorter in height than at least some portions of the cover112. Mounting base 113 includes a perimeter frame 115 which may have anoverall shape in top plan view which substantially conforms with thecorresponding shape of the cover 112 of shroud 100.

The bottom 108 of shroud 100 may include open areas and closed areas.Shroud 100 may therefore further include a horizontal partition wall116. In two-piece constructions of the shroud 100 described above, thehorizontal front wall 116 may be formed in lower mounting base 113. Insome shrouds, partition wall 116 may define a generally circular shapedcentral aperture 109 (in top plan view) which is configured anddimensioned to be concentrically aligned with a rotational axis of fanimpeller 31 when the shroud 100 is mounted on the blower housing 40. Insome shrouds, central aperture 109 may have a diameter which is at leastthe same or larger than a diameter or outer side 34 of the impeller 31so as to not impede inlet cooling air flow into impeller 31. Thecircular aperture 109 with its center positioned at the intersection ofthe longitudinal axis LA and a transverse axis TA as shown in FIG. 21may be considered to define two front quadrants Qf and two rearquadrants Qr of the shroud 100 for convenience of reference indescribing additional features of the shroud hereafter.

Shroud 100 may further include at least two enlarged and horizontallyelongated air inlet passages 110 and associated air inlet ports 107. Theair inlet passages 110 are configured and operable to attenuate fannoise. In addition to sound attenuation, the air inlet passages 110 andports 107 are further operable via rotation of the fan impeller 31 todraw outside ambient cooling air underneath the shroud and inwardstowards the impeller 31.

Air inlet passages 110 and ports 107 collectively define correspondinghorizontally elongated openings which may be formed from rear portionsof the shroud peripheral sidewalls 105, adjoining closed top wall 104,and the downwardly open bottom 108 of the shroud 100. The air inletpassages 110 may have a generally inverted U-shape in cross-sectiontaken transversely to the inlet air flow path.

The peripheral sidewalls 105 of the shroud 100 may define a plurality ofangled interior surfaces 105 a which are acoustically configured,designed, and placed to induce internal reflection and capture of noiseproduced by the fan 30. The interior surfaces 105 a within the air inletpassages 110 may form adjoining multi-faceted angled surfaces intendedto reduce the amount of noise which escapes through the air inlet ports107. In one shroud, the angled interior surfaces 105 of the shroud 100are designed to direct a majority of the sound waves generated by thefan impeller 131 back towards the center of the shroud.

In one configuration of the shroud 100, a majority portion of each airinlet passage 110 may be positioned primarily in one of the two opposingrear quadrants Qr of the shroud (e.g. rear of the transverse axis TA)proximate to the rear 102 of the shroud body and adjoining rearwardportions of sides 103 in each of these quadrants. The air inlet passages110 may be located at these rear side portions of shroud 110 whichcorrespond to low (or in some cases the lowest) sound pressure wavepositions in comparison to other portions of the shroud, as determinedby computer aided modeling. Accordingly, escaping noise levels from thecooling air system fan 30 from beneath the shroud 100 are at theirlowest at the air inlet ports 107 in these rear quadrant positions.

As shown in FIGS. 19-24, air inlet ports 107 may be angled to face in agenerally downwards and outwards direction towards the rear 102 ofshroud 100 for radiating noise generated by fan 30 (or other enginecomponents) rearwards away from the operator generally seated forward ofthe engine 26 in some outdoor riding equipment configurations (see, e.g.FIGS. 1A and 1B). The directional sound arrows in FIG. 1B show a generalemission direction of the fan noise escaping the shroud through the airinlet ports 107 (radiated noise is very complete in this frequencyrange; these arrows are meant for general illustration purposes).

In some systems, one or more fins, dividers, or separating barriers maybe placed within the air inlet ports 107, the air inlet passages 110, orboth to serve multiple functions. For example, the fins may act todirect or guide the inlet air into the blower housing 40. The fins mayguard the air inlet ports 107 from receiving grass or other debris intothe housing 40. The fins may also or alternatively be constructed orengineered to force noise wave propagation in a certain direction out ofthe shroud 100. Other variations are possible.

The air inlet passages 110 each may define a respective centerline CLextending along the greatest length of the passages from a common pointof intersection (origin) proximate to the front 101 of shroud 100 to therear of the passages as shown in FIG. 15. The air inlet passages 110 maybe disposed at an angle A1 with respect to the longitudinal axis LAextending from front 101 to back 102 of shroud 100. In some shrouds,angle A1 may be without limitation between 0 and 90 degrees.Accordingly, the air inlet passages 110 may be angled and sweptrearwards on shroud 100 having a somewhat wing-like configuration in topplan view. The air inlet passages 110 may be laterally spaced apart fromeach other by an angle equivalent to two times angle A1. The air inletports 107 associated with air inlet passages 110 may further be disposedat an angle A2 to the horizontal plane defined by the bottom 108 of theshroud 100 (see, e.g. FIG. 22) to direct fan noise not only downward butalso outwards from the rear of the engine 26. In some shrouds, angle A2may be without limitation between 0 and 90 degrees.

Air inlet passages 110 may be horizontally elongated from front to rearin the direction of the longitudinal axis LA and extend rearward by adistance farther than a central rear portion of the rear 102 of theshroud closest to central aperture 109 than the terminal ends 117 ofeach as shown. The air inlet passages 110 are shaped to direct emittedfan noise from the fan 30 rearwards and generally downwards away fromthe operator's ears. In addition, the noise from the fan is directed byand within the air inlet passages 110 along the same pathway as theinlet cooling air drawn inwards towards the fan 30, but in the oppositedirection to the incoming air. The drawing of intake air inwards in adirection opposite the direction of propagating sound waves mayattenuate, damp, or otherwise reduce a level (or volume) of noise whichis emitted through the air inlet ports 107.

It should further be noted that the placement and configuration of thehorizontal partition wall 116 is intended to preclude cooling air intakeinto the shroud 100 and blower housing 40 at shroud locations which aremore proximate to the operator (see, e.g. FIGS. 1A and 1B), and hencecorrespondingly which provide a possible directional pathway for fannoise to escape in the direction towards and reach the operator's ears.Accordingly, cooling air inflow into the shroud 100 may be restricted toeach of the two air inlet ports 107 located at the distal rear end 102of the shroud by partition wall 116 (see, e.g. FIGS. 19-24) rather thanproximal portions of the shroud closer to the operator. Cooling systemnoise emissions may therefore be substantially restricted to the tworear quadrants Qr of shroud 100.

The foregoing partially enclosed configuration, elongated shape, andgeometry of surfaces inside each air inlet passages 110 collectivelyhelps induce internal reflection of the sound waves generated by fan 30within each air inlet passages 110, thereby capturing a portion of thesound to reduce the overall noise level (e.g. measured in decibels ordBA) emitted from the air inlet passages that reaches the operator. Theplacement of the air inlet passages 110 in the two rear quadrants Qr ofthe shroud 100 most distal to an operator and directional angledpositioning of the air inlet ports 107 described above substantiallydirects a significant amount of the fan noise escaping from the inletair passages away from the operator positioned generally forward of theengine 26, as shown in FIGS. 1A and 1B. This reduces the overall coolingair system (and other engine component) sound level at the operator'sears. The placement of the air inlet passages 110 and associated airinlet ports 107 as described herein provides maximum attenuation ofsound pressure waves in a direction away from the operator.

It will be appreciated that the shroud 100 could be located andpositioned at various other locations with respect to or covering theentrance of a cooling system for the engine. Accordingly, the shroud isnot limited to the placement and orientation shown and described hereinby way of the non-limiting examples presented.

In other possible configurations of shroud 100, it will be appreciatedthat the shroud body may one-piece of unitary construction with anintegral cover 112 and mounting base 113 which is attachable to theblower housing 40.

In some variations of the shroud, noise insulating material such assound damping fibrous material may be applied inside cavity 106 ofshroud 100 to increase overall noise reduction performance of shroud100. The sound damping fibrous material may, for example, be afiberglass absorptive material, a foam material such as melamine,damping felt, or various other materials. The sound damping fibrousmaterial may be applied to various areas within the cavity 106, such ason the underside of the top wall 104 and/or inside of verticalperipheral sidewalls 105. Other variations are possible.

According to another aspect of the present disclosure, the noisesuppression shroud 100 may include one or more quarter wave resonator120. Quarter wave resonators 120 may further reduce the level of noiseemitted by the engine cooling air system to the ambient environment.Quarter wave resonators (QWR) may attenuate sound via acoustic wavecancellation, which in the present case may be noise frequenciesgenerated by the fan 30 or other engine components.

Referring primarily to FIGS. 14, 17, and 18, quarter-wave resonator 120in one shroud includes an array of multiple cells 121 formed byadjoining and/or intersecting grid partition members 122. Partitionmembers 122 may be disposed inside internal cavity 106 of shroud 100. Insome shroud configurations, the partition members 122 may be formedintegrally with the shroud 100 as a unitary structural part of theshroud top wall 104 and/or vertical peripheral sidewalls 105. Ininstances where the shroud 100 may be formed of a polymer or plastic,partition members 122 may be integrally molded with the shroud. In othershroud configurations, partition members 122 may be separate elementswhich are insertable into and attachable to the shroud 100 as either apreassembled unit or as individual partition members 122 each separatelyattachable to the shroud. The partition members 122 may be attached toshroud 100 by any suitable method or combinations of methods includingwithout limitation fasteners, snap fit, frictional fit, adhesives,welding, brazing, etc.

The partition members 122 may be configured and arranged to formcorresponding cells 121 having any suitable polygonal or other shapedesired (in bottom plan view), including for example without limitationsquare (as shown), rectangular, triangular, hexagon, octagon, circular,honeycomb, and others. Partition members 122 may have any suitabledimensions in both length Lp and width Wp (in bottom plan view), and inheight Hp (in side elevation view) as shown for example in FIG. 14. Theheight Hp forming a distance between the bottom edge 123 and inside oftop wall 104 of the shroud 100 defines a corresponding cell depth Dc forcells 121 (see, e.g. FIG. 18). In one shroud, the partition members 122may have height Hp selected so that the bottom edge 123 of the partitionmembers 122 is spaced vertically apart from the top 43 of the blowerhousing 40 to form a gap that avoids impeding the inflow of cooling airinto the impeller 131.

The height Hp of partition members 122 may be different in variousportions on the underside of shroud top wall 105 so that the cells 121may have different depths Dc. This may be accomplished by configuringthe top wall 104 differently in various areas of the shroud todecrease/increase the, or alternatively by adding intermediatehorizontal walls (not shown) in various areas beneath the shroud. Forexample, in systems where the top wall 104 is slightly curved, thecurved nature of the top wall 104 may create cells 121 with differentdepths Dc. Accordingly, in some shrouds, the partition member 122 heightHp and corresponding cell depth Dc may be either non-uniform or uniformdepending on the intended sound frequencies to be attenuated by thequarter wave resonator 120.

The frequency of noise that may be reduced (by wave cancellation)through the use of quarter wave resonators 120 (and cells 121) maydepend, at least in part, on the depth Dc of the cells 121. The depth Dcof the cell 121 may be tuned to reduce (or cancel) noise at a certainfrequency (or frequency band). In some quarter wave resonators 120, somecells 121 may be configured to have different depths Dc such that somecells 121 may reduce (or cancel) noise at different frequencies thanother cells 121. For example, as discussed, in systems where the topwall 104 is slightly curved (or otherwise not strictly horizontal), thecells 121 below the top wall 104 may have difference depths Dc. As such,the aggregate result may be that the quarter wave resonator 120 may beused to reduce (or cancel) noise at a wider range of frequencies.

At least a portion of shroud 100 may include the quarter wave resonator120 with associated partition members 122. In some shrouds, thepartition members 122 may be concentrated towards the geometric centerof the shroud 100 opposite the fan impeller 131 to attenuate noiseemitted from the impeller. In other shrouds, various discrete portionsof the cavity 106 within shroud 100 may include quarter wave resonators120 with partition members 122 (e.g. opposite impeller, in portions ofair inlet passages 110, etc.). In yet other shroud configurations, asshown in FIG. 27, substantially the entire cavity 106 may be filled bythe quarter wave resonator 120 and partition members 122 to the extentpermitted by the shroud geometry.

The quarter wave resonator 120 may be tuned for abating cooling airsystem noise within a specific range or band of frequencies by varyingdesign parameters such as without limitation the extent of the shroud100 which includes a quarter wave resonator 120, shape of the cells 121formed by the partition members 122, depth of cells Dc, and materials ofconstruction of the partition members 122. The sound attenuationperformance of the shroud 100 may therefore be optimized by such tuningto compensate for and reduce the specific noise generation frequenciesof a given engine system. Accordingly, the quarter wave resonator 120may be configured and tuned to remove a narrow band or a broad band ofnoise frequencies.

In some shrouds, the quarter wave resonator 120 may be omitted as shownin FIG. 26 and the shroud 100 may rely on the air inlet passages 110 toattenuate system noise.

The shroud 100 (including base 113 and cover 112) and quarter waveresonator 120 may be made of any suitable metallic or non-metallicmaterials, including without limitation metals such as steel oraluminum, polymers/plastics (e.g. polyvinylchloride, acrylic, etc.),fiberglass, and others. In one example, the shroud 100 may be made of20% glass filled polypropylene. The quarter wave resonator 120 partitionmembers 122 may be made of the same or different material. The blowerhousing 40 in one example may be made of the same 20% glass filledpolypropylene or another suitable material. Accordingly, the shroud,quarter wave resonator, and blower housing are not limited by materialsof construction which are selected to provide the desired soundabsorption characteristics and other performance factors as appropriateto suit a particular application.

According to another aspect of the present disclosure, the noisesuppression shroud 100 may include a micro-perforated panel (MPP) 130for sound absorption in addition to or instead of quarter wave resonator120. FIGS. 28 and 29 show a shroud 100 incorporating a micro-perforatedpanel 130 used in conjunction with a quarter wave resonator 120. Themicro-perforated panel may be comprised of a substantially flat sheet131 of material (e.g. metal) which includes a plurality of regularlyspaced apart micro-sized pores or holes 132 of a predetermined diameterand pitch P (spacing between adjacent holes). The holes 132 may have thesame diameter or non-uniform diameters, and be any suitableconfiguration including circular as commonly used or other shapes.

The micro-perforated panel 130 may be positioned at various locationswithin the shroud 100. The micro-perforated panel 130 may divide theshroud 100 into two or more separate cavities. For example, themicro-perforated panel 130 may be positioned horizontally through theshroud 100, dividing the shroud into a top cavity and a bottom cavity.In some such systems, the micro-perforated panel 130 may be positioned adepth Dp from the top wall 104 that is engineered or tuned to providewave cancelation of certain undesirable noise frequencies, and/or suchthat the top wall 104 is positioned at a distance of lowest wavepressure from the micro-perforated plate 130. The micro-perforated panel130 may be planar, or may have a curved, rippled, bent, or othersurface. Other variations are possible.

The micro-perforated panel 130 may be positioned below the quarter waveresonator 120 between the bottom 108 of shroud 100 and the quarter waveresonator. In other shrouds, the micro-perforated panel 130 may bepositioned above the quarter wave resonator 120 between top wall 104 ofshroud 100 and the quarter wave resonator. An air-space C having a depthDp may be formed behind the micro-perforated panel 130 below the topwall 104 of shroud 100. In this particular example, the depth Dp of theair space C may be coextensive with the height Hp of the partitionmembers 122 and depth Dc of shroud 100 in the quarter wave resonator120. Air space C associated with the micro-perforated panel 130 willaccordingly be formed from a portion of the overall shroud cavity 106.

In one configuration of shroud 100, the micro-perforated panel 130 mayenclose the entire bottom 108 of the shroud as shown. In other possibleshrouds, the micro-perforated panel 130 may cover only portions of thebottom 108 of the shroud 100 such as over the areas which include aquarter wave resonator 120, or alternatively areas of the shroud that donot include quarter wave resonators.

Micro-perforated panels are effective for absorbing sound or noisewithin a predetermined attenuation frequency band or range based on theHelmholtz resonance principle, thereby reducing the resultant reflectedsound. The attenuation frequency band may be customized to be narrow orwide by varying the design parameters of the micro-perforated panel. Thepore or hole 132 size, spacing or pitch P, thickness Tp of the sheet131, material of construction of sheet 131, and depth Dp of the airspace C behind the sheet all affect the resultant noise cancellationproperties of a micro-perforated panel and attenuation frequencies.Accordingly, the inventors have discovered that these parameters can beadjusted to change the noise cancellation characteristics of themicro-perforated panel 130 and tune the micro-perforated panel forfiltering out specific fan frequencies to suit a given engine andassociated cooling air system at hand. In some shrouds, the depth of Dpof air space C can be increased as desired by making the top wall 104 ofthe shroud domed or convex shaped as shown by the dashed top wall 104′in FIG. 29. These foregoing parameters may be adjusted to achieve thedesired sound frequency filtering and attenuation characteristics fornoise reduction.

In some systems, one or more of the hole 132 size, spacing or pitch P,and/or thickness Tp of the sheet 131 may vary within the samemicro-perforated panel 130. For example, holes 132 near the center ofthe micro-perforated panel 130 may be sized differently from the holes132 a larger radial distance from the center of the micro-perforatedsheet 130. In this example, the holes 132 near the center of themicro-perforated panel 130 may enable or cause the micro-perforatedpanel 130 to absorb noise around a first frequency range (tuned to theparameters of the holes 132 at the center of the micro-perforated panel130) near the center of the panel 130, while the holes 132 near theperimeter of the micro-perforated panel 130 may enable or cause themicro-perforated panel 130 to absorb noise around a different frequencyrange (tuned to the parameters of the holes 132 near the outer edges ofthe micro-perforated panel 130). Other variations are possible.

Referring to FIG. 29, the shroud 100 with micro-perforated panel 130 mayalso include partitions which in some designs may be configuredsimilarly to the partition members 122 shown provided for the quarterwave resonator 120. The partition members 122 in such shrouds 100 may beconstructed, positioned, and/or used to force a certain wave propagation(such as a linear plane wave propagation) between the micro-perforatedpanel 130 and the top wall 104. The forced wave propagation created bythe partitions 122 may increase the noise attenuation and absorptioncharacteristics of the shroud 100. The partitions for themicro-perforated panel 130 may or may not also behave as a quarter waveresonator, tuned for wave cancelation of certain frequencies of noise.The micro-perforated panel 130 may be positioned above, or below, thepartition members 122.

As shown in FIG. 30, more than one micro-perforated panel 130 may beused to broaden the range of frequencies absorbed by the panel. In theshroud 100 shown, two micro-perforated panels 130 and 130′ arevertically arranged next to each other, and separated by an air gap. Inother variations, the two panels 130 and 130′ may be stacked together incontact with each other. Each of the panels 130 and 130′ may havedifferent sound absorption characteristics by providing different hole132 size, spacing or pitch P, thickness Tp of the sheet 131, ormaterials of construction of the sheet for each panel. Accordingly, asystem with two panels 130 and 130′, each with different soundabsorption characteristics, may absorb sound at a wider range offrequencies than a system with only one panel 130. In other variations,the sheets 130 and 130′ may be identical. Additionally, the air gapbetween the two micro-perforated panels 130 and 130′ may be constructedsuch that the distance between the two panels 130 and 130′ providesadditional wave cancelation and/or low wave pressure properties. Due tothe construction and configuration of the spacing, acoustic cancelationmay occur as sound/noise waves reflect between the panels 130 and 130′and also are re-directed back towards matching waves. Sound waves inopposite directions with equal or close frequencies will tend to canceleach other (attenuation).

As also shown in FIG. 30 and noted above, one or multiplemicro-perforated panels 130, 130′, etc. may be used alone withoutquarter wave resonator 120. It will be appreciated, however, thatmultiple micro-perforated panels 130 may also be used with a quarterwave resonator 120.

In one example of a micro-perforated panel 130, the holes may have adiameter ranging from and including 0.05 mm to 0.5 mm. The holes may beformed by any suitable method, including without limitation lasercutting or other suitable methods. The micro-perforated panel sheet 131may be made of any suitable metallic or non-metallic materials,including without limitation metals such as steel or aluminum,polymers/plastics (e.g. polyvinylchloride, acrylic, etc.), fiberglass,and others. Accordingly, micro-perforated panel 130 is not limited bymaterials of construction which are selected to provide the desiredsound absorption characteristics suited for a particular application.

In some variations of a micro-perforated panel, the peripheral edges ofmicro-perforated panel 130 may be sealed to the inside of shroud 100along vertical sidewalls 105 to create a substantially air tight airspace C between the shroud and panel to minimize reflected sound leakagebetween the panel edges and the shroud. Reflected noise or sound fromair space C behind the panel will therefore only have a pathways backout through the panel holes 132. The edges of micro-perforated panel 130may be sealed by any suitable method including without limitationcaulking or sealants, gaskets, welding (e.g. metal or sonic for plasticsdepending on the materials used for the shroud and panel), and others.

The inventors conducted predictive computer modeling of the shroud 100to determine the potential sound transmission loss which could beachieved by various combinations of a shroud with and without some ofthe foregoing noise suppression features disclosed herein. The resultanttransmission loss curves are shown in FIG. 39. The baseline curveresults (light-weight dashed line) represents an empty shroud and airinlet passages 110 without quarter wave resonator or micro-perforatedpanel, thereby relying on only the cooling air passages and shroud bodyfor sound attenuation. The addition of a quarter wave resonator 120 wasmodeled having a 9×9 cell array (9 chambers as identified in FIG. 39) asdescribed herein (light-weight sold line curve) to determine its effecton noise suppression performance of the shroud. The effect of adding amicro-perforated panel 130 was modeled both alone in the shroud 100(heavy-weight solid line curve) and in combination with the 9×9 cellquarter wave resonator 120 (heavy-weight dashed line curve).

As seen in the results of this modeling, the noise suppressionperformance (i.e. highest decibel sound transmission loss) of a shroud100 incorporating micro-perforated panel 130 either alone or withquarter wave resonator 120 was generally better over a wide band orrange of frequencies than shrouds without the micro-perforated panel.The addition of a quarter wave resonator alone also demonstratedgenerally better performance than an empty shroud. It will beappreciated, however, that even the empty shroud 100 incorporating thespecially configured and positioned air inlet passages 110 providesimproved noise reduction and isolation performance, both of which may beeven further improved through the use of fibrous absorptive materials.The results of this modeling further demonstrates that the shroud andnoise suppression features disclosed herein are each highly customizablefrom a noise suppression standpoint and may be combined in variouscombinations to achieve a desired sound attenuation levels at variousfrequency bands or ranges of interest for a given application.

In view of the foregoing discussion and computer-aided modeling, it willbe appreciated that the shroud 100 structure itself with air inletpassages 110 may be considered to provide a baseline noise reductionbeing tuned to actively reduce fan noise within a certain firstfrequency range or band and degree of noise reduction (i.e. decibel orsound pressure). A quarter wave resonator 130 or micro-perforated panel130 may be added which functions to reduce noise in a second frequencyrange or band which in concert with the air inlet passages 110 have acumulative noise reduction effect. For systems with micro-perforatedpanels 130, partitions 122 may be added to provide a forced linear wavepropagation that may further reduce noise of the system. The remainingone of the quarter wave resonator 120 or micro-perforated panel 130 notused may, in some systems, be added which functions to reduce noise in athird frequency range or band have a further cumulative noise reductioneffect. Any of these systems may also include fibrous absorptivematerial which may be constructed to provide attenuation over a desiredfrequency range based on the absorptive coefficient of the fibrousmaterial.

Any of the first, second, or third frequencies ranges may be the same,effecting an increased noise reduction over that frequency range. Forexample, a shroud may include a micro-perforate panel 130 constructed toabsorb sound at a frequency range of 800 Hz to 1000 Hz, while thequarter wave resonators 120 may be constructed with a depth Dc to cancelwaves in the same or an overlapping frequency range. In other examples,the first, second, or third frequency ranges may be different to reducenoise over a wider frequency range than either range individually. Thecombined reduction of fan noise by employing some or all of theforegoing sound reduction features may therefore operate to providesignificant or maximum noise reduction over a desired and focusedspectrum of frequencies, and/or attenuate sound over a wide spectrum offrequencies thereby providing a high degree of customization to thenoise suppression system described herein.

According to another aspect of the prevent disclosure, amicro-perforated panel 130 may be cooperatively designed in conjunctionwith the type of fan impeller selected to optimize the performance ofthe shroud noise reduction system. The mono-pitch impeller 31 (equalcircumferential blade spacing) or modulated impeller 33 (unequalcircumferential blade spacing) designs each have different noisegeneration characteristics. For example, mono-pitch impellers 31 maytypically produce the greatest levels of noise at a narrow (andsometimes higher) frequency bands than the modulated impeller 33 design.With either design, the blade spacing and configuration of the impellermay be selected to intentionally constrain the greatest noise levels towithin a predetermined frequency range which coincides with thefrequency range for which a micro-perforated panel 130 has been designedto attenuate those same frequencies. For example, an engine 26 may havea mono-pitch (equal blade spacing) impeller 31 which was intentionallydesigned to generate the greatest level of noise within a first band offrequencies from about 1040 Hz to 1560 Hz. Impeller noise fallingoutside of this range will be lower and may be at acceptable levels insome instances. The micro-perforated panel 130, through manipulating itsdesign parameters as described above (e.g. hole spacing, pitch, panelthickness, etc.), may then be specifically designed to have the noisesuppression characteristic of operably attenuating sound falling withinthe same band of frequencies as the impeller from about 1040 Hz to 1560Hz over a given engine speed. The end result is attenuation of impellernoise over a relatively wide range or band of frequencies includingminimizing the most offensive peak frequencies of the impeller.Accordingly, while the use of a mono-pitched impeller 31 may otherwisebe undesirable due to the increased noise at a narrow frequency band,the use of micro-perforated plates 130 and/or quarter wave resonators120 tuned to reduce (through absorption or wave cancellation) noisewithin that frequency may result in a quieter engine than one with amodulated-pitch impeller 33.

While the foregoing description and drawings represent some examplesystems, it will be understood that various additions, modifications andsubstitutions may be made therein without departing from the spirit andscope and range of equivalents of the accompanying claims. Inparticular, it will be clear to those skilled in the art that thepresent invention may be embodied in other forms, structures,arrangements, proportions, sizes, and with other elements, materials,and components, without departing from the spirit or essentialcharacteristics thereof. In addition, numerous variations in themethods/processes. One skilled in the art will further appreciate thatthe invention may be used with many modifications of structure,arrangement, proportions, sizes, materials, and components andotherwise, used in the practice of the invention, which are particularlyadapted to specific environments and operative requirements withoutdeparting from the principles of the present invention. The presentlydisclosed embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingdefined by the appended claims and equivalents thereof, and not limitedto the foregoing description or embodiments. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the invention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

What is claimed is:
 1. A noise suppression shroud for an engine having ablower housing and fan positioned therein, the shroud comprising: alongitudinal axis; a body having a lower portion mountable on the blowerhousing of the engine and an upper portion, the body further comprisinga top wall and extending downwards therefrom a front wall, a rear wall,and opposing first and second lateral sidewalls; a cavity formed in thebody; the first and second lateral sidewalls each defining a discretefirst air inlet passage and a discrete second air inlet passagerespectively, the first and second air inlet passages each extendingrearwards beyond the rear wall and on opposite sides of the rear wall,the first and second air inlet passages being downwardly open and influid communication with the fan in the blower housing; a quarter waveresonator disposed in the cavity and tuned to attenuate noise within afirst range of frequencies generated by the fan within the blowerhousing; wherein each of the air inlet passages defines a centerlinewhich is angled obliquely to the longitudinal axis, the air inletpassages each including a rear terminal end which is spaced fartherapart laterally from the longitudinal axis than a front end of the airinlet passage which is connected to the front wall of the body of theshroud; wherein the first and second air inlet passages are eachoperable to separately draw cooling air underneath the shroud andinwards in a forward direction towards the fan, the cooling air drawn inby the first and second air inlet passages being combined in the cavityand collectively drawn into the fan.
 2. The shroud of claim 1, whereinthe body of the shroud comprises a closed top and a partially openbottom.
 3. The shroud of claim 1, wherein the quarter wave resonatorcomprises a plurality of intersecting partitions forming a plurality ofcorresponding cells.
 4. The shroud of claim 3, wherein the partitionsare disposed in the upper portion of the body of the shroud.
 5. Theshroud of claim 3, wherein bottom edges of the partitions are verticallyspaced apart from a bottom of the body of the shroud.
 6. The shroud ofclaim 1, wherein the quarter wave resonator is vertically aligned with afan impeller positioned in a top of the blower housing.
 7. The shroud ofclaim 1, further comprising at least one micro-perforated panel disposedin the shroud, the at least one micro-perforated panel being tuned toreduce noise within a second range of frequencies.
 8. The shroud ofclaim 7, wherein the at least one micro-perforated panel is disposedbelow the quarter-wave resonators in the cavity of the body of theshroud.
 9. The shroud of claim 7, further comprising a secondmicro-perforated panel disposed in the shroud, the secondmicro-perforated panel being tuned to remove noise within a third rangeof frequencies.
 10. The shroud of claim 1, wherein a majority of eachair inlet passage is positioned in one of two rear quadrants defined bythe shroud on opposite sides of the longitudinal axis.
 11. The shroud ofclaim 1, wherein the lower portion of the body is a base configured forremovable mounting on a blower housing and the upper portion is aseparate cover removably attachable to the base.
 12. The shroud of claim11, wherein the base defines a pair of air inlet ports in fluidcommunication with and complementary configured to the first and secondair inlet passages.
 13. The shroud of claim 1, wherein the rear terminalends of each air inlet passages extend rearwards beyond the rear wall ofthe body of the shroud.
 14. A noise suppression system for an engine,the system comprising: a shroud mounted on a blower housing associatedwith the engine and having a fan positioned therein; the shroudcomprising a longitudinal axis, a top wall and extending downwardstherefrom a front wall, a rear wall, and opposing first and secondlateral sidewalls; a cavity formed in the shroud and positioned over thefan; the first and second lateral sidewalls each defining a discretefirst air inlet passage and a discrete second air inlet passagerespectively, the first and second air inlet passages each extendinglongitudinally rearwards beyond the rear wall and on opposite sides ofthe rear wall, the first and second air inlet passages being downwardlyopen and in fluid communication with the fan in the blower housing; atleast one micro-perforated panel disposed in the shroud, the at leastone micro-perforated panel being tuned to reduce noise generated by thefan within a first range of frequencies; wherein each of the air inletpassages defines a centerline which is angled obliquely to thelongitudinal axis, the air inlet passages each including a rear terminalend which is spaced farther apart laterally from the longitudinal axisthan a front end of the air inlet passage which is connected to thefront wall of the body of the shroud; wherein the first and second airinlet passages are each operable to separately draw cooling airunderneath the shroud and inwards in a forward direction towards thefan, the cooling air drawn in by the first and second air inlet passagesbeing combined in the cavity and collectively drawn into the fan. 15.The noise suppression system of claim 14, further comprising a secondmicro-perforated panel disposed in the shroud, the secondmicro-perforated panel being tuned to remove noise within a second rangeof frequencies.
 16. The noise suppression system of claim 14, furthercomprising a quarter wave resonator disposed in the cavity and tuned toreduce noise within a third range of frequencies.
 17. A noisesuppression system for an air cooled engine, the system comprising: ashroud mounted on a housing of a cooling air fan associated with theengine, the shroud comprising a longitudinal axis, a top wall andextending downwards therefrom a front wall, a rear wall, and opposingfirst and second lateral sidewalls; a cavity formed in the shroud andpositioned over a fan impeller, the fan impeller rotationally supportedby the housing of the cooling air fan; and the first and second lateralsidewalls each defining a discrete first air inlet passage and adiscrete second air inlet passage respectively, the first and second airinlet passages each extending longitudinally rearwards beyond the rearwall and on opposite sides of the rear wall, the first and second airinlet passages being downwardly open and in fluid communication with thefan in the blower housing; the first and second air inlet passages eachbeing formed in a rear quadrant of the shroud on opposite sides of thelongitudinal axis; wherein the first and second air inlet passages areeach operable to separately draw cooling air underneath the shroud andinwards in a forward direction towards the fan, the cooling air drawn inby the first and second air inlet passages being combined in the cavityand collectively drawn into the fan; wherein each of the air inletpassages defines a centerline which is angled obliquely to thelongitudinal axis, the air inlet passages each including a rear terminalend which is spaced farther apart laterally from the longitudinal axisthan a front end of the air inlet passage which is connected to thefront wall of the body of the shroud; wherein the air inlet passages areacoustically configured to attenuate noise generated by the cooling airfan.
 18. The noise suppression system of claim 17, wherein the air inletpassages include a plurality of angled sidewall surfaces configured forreflecting noise generated by the fan, each of the angled sidewallsurfaces being disposed at an oblique angle to an adjoining angledsidewall surface.
 19. The noise suppression system of claim 17, whereinthe shroud includes a closed top wall.
 20. The noise suppression systemof claim 17, wherein the shroud has a closed bottom except for the openair inlet ports formed by the air inlet passages and a central openingpositioned above the fan impeller.
 21. The noise suppression system ofclaim 17, wherein a majority of each air inlet passage is positioned inone of the two rear quadrants defined by the shroud.